The invention concerns the use of polyethylene terephthalate for the production of transparent packaging containers which are formed by means of a welded longitudinal seam, in particular packaging tubes, and a tube formed from that plastic material.
Packaging containers, in particular transparent bottles of polyethylene terephthalate (identified by the abbreviation PET) are known, which have experienced a significant spread, by virtue of the good barrier action of the PET. Packaging containers are produced from the specified material by a procedure whereby a material blank is firstly injected in the form of a semimanufactured article. That blank is heated in the processing operation and is introduced into a mould which corresponds to the form of the container and in which the blank is inflated until it completely reproduces the shape of the container. Bottles produced in that way for example from the blankxe2x80x94also referred to as blow-moulded bottlesxe2x80x94are as clear as glass (referred to for the sake of brevity as transparent) and do not have any flow seams or similar adverse surface effects caused by the moulding procedure. Besides transparent PET-bottles, transparent packaging containers of PET in tube form are also known. For the production thereof, in the same way as in the case of bottles, firstly a blank is injected, and the blank is heated upon further processing, but in contrast to the production of bottles, the heating operation is not effected in a blow mould but in a deep-drawing or stretching tool, in a procedure whereby the blank is converted in shape to constitute a tube which can be filled at one end, comprising a tube body with a tube head moulded thereon. Transparent tubes produced in that way however, in contrast to bottles, suffer from the disadvantage of adversely affected transparency which can no longer be considered to be as clear as glass, in accordance with the quality feature involved. The reason for this lies in changes in the crystalline structure as a result of the deep-drawing and stretching procedure. The most serious disadvantage however is the fact that tubes produced by deep drawing and stretching can only be decorated at high cost in terms of printing procedures. Inevitably packaging containers produced in that way have to be decorated or printed upon, while rotating, in a condition of being carried on a bar or mandrel, this being in contrast to printing on a foil strip in a flat condition, which is subsequently converted into the shape of a tube body. In comparison therewith, packaging containers, in particular tubes, which are produced by deep-drawing and stretching enjoy the advantage of so-called xe2x80x9chandling neutralityxe2x80x9d. The term xe2x80x9chandling neutralityxe2x80x9d is used to denote an impression of feel in respect of the container, which is unimpaired on the part of the user. A container does not enjoy xe2x80x9chandling neutralityxe2x80x9d for example when regions of a container where it is felt or gripped have perceptible surface deformations or impairments, for example welded seams, portions corresponding to the shape of joins in moulds, and so forth. From the point of view of the packer using packaging containers and in particular packaging tubes, xe2x80x9chandling neutralityxe2x80x9d, besides the mechanical-technological properties of a packaging container, is a quite essential quality feature which is customer-oriented from the point of view of the packager.
Tubes of plastic materials are known, the tube bodies of which, taking strips of plastic foil as the basic starting point, are formed by longitudinal shaping of the strip to form a tube and welding of mutually superposed edge portions of the foil strip. Tubes of that kind are referred to as longitudinal seam-welded tubes. Those tubes can be decorated at a comparatively low cost in terms of printing procedure, insofar as the foil strips can be printed upon while in a condition of lying flat, before being formed into a tube. They suffer from the disadvantages however that longitudinal seam-welded tubes naturally cannot entirely attain the quality feature of xe2x80x9chandling neutralityxe2x80x9d, and the welded seam as such remains an area which suffers certain reservations from the point of view of the engineer dealing with the plastic materials and the man skilled in the art of plastic welding, depending on the nature of the respective plastic materials used. Those reservations arise out of possible variations in the material of the seam in comparison with that of the rest of the wall of the tube, a tendency on the part of the material which is to be welded to being squeezed out during the welding operation, that is to say, a tendency to suffer from so-called flake-like extrusion phenomena which contaminate the packaged material, a freedom from cracks, flaws or tears between the seam and the adjoining wall of the tube body, an unequal diffusion barrier effect on the part of the seam and the rest of the material of the wall of the tube, and also a shrinkage characteristic on the part of the seam, which is difficult to determine and which is generally governed by the rate of manufacture, after fusing, pressing and cooling of the plastic material at the edge portions of the foil, such operations being effected to produce the seam. Those reservations are empirically ascertained in regard to individual plastic materials as such and in terms of their individual effects, but generally not in regard to their combinational effects, that is to say if two or more of the above-mentioned technical limitations occur at the same time, which has the result that the average man skilled in the art who is concerned with the development and production of longitudinal seam-welded plastic tubes does not undertake replacing or exchanging a tried-and-tested plastic material by a new plastic material which is unknown in terms of its behaviour, without overcoming major technical prejudices, and that is the case all the more insofar as the effects of the reservations in part occur only after a time delay and sometimes only after the tube has been filled with packaging material.
Taking transparent packaging containers, produced in particular from PET, by deep-drawing and stretching, as the basic starting point, the invention aims to provide for the production of an identical packaging container, namely a packaging tube, by means of longitudinal seam welding, and determining a plastic material which can be used therefor, that is to say the invention sets forth the problem which is solved by means of the features of claim 1.
European patent specification No 0 496 704 discloses a longitudinal seam-welded packaging tube, the tube body of which is formed from three-layer plastic laminate. The laminate includes a central layer of PET lined on both sides with a layer of a polyethylene, wherein the central layer of PET performs the function of a barrier layer and the polyethylene layers form the welded seam as polyethylene has significantly better weldability than PET. EP 496 704 teaches the man skilled in the art of plastic tube production that PET can be used as a component of a multi-layer laminate, but identifies this teaching in comparison with other plastic materials with weldability which cannot be readily accomplished, so that to solve the problem of the present invention, having regard to the reservations which are crucial to him, the man skilled in the art does not readily turn to use of the claimed tube material.
The use of the material according to the invention for the production of packaging tubes by means of longitudinal seam welding makes it possible to produce tubes of the same transparency as blow-moulded bottles, which in terms of colour can be decorated and also embossed at comparably low expenditure in terms of printing procedure (printing a flat article before it is formed into a tube). Handling neutrality is not entirely but approximately achieved insofar as the seam affords a surface structure which blurs for the user of the tube the feel impression of a technically governed surface impairment. Surprisingly the welding operation which in itself is technically demanding did not result in intolerable changes in material and squeezing-out phenomena, that is to say extrusion effects; in addition there were no problems in regard to freedom from cracks, flaws and tears and differences in diffusion barrier effects. Furthermore, contrary to expectation, post-shrinkage occurred to such a slight degree that distortion of the welded tube body to form a xe2x80x9cbanana-likexe2x80x9d shape, as is frequently to be encountered when using plastic materials, is negligible. In addition, a higher output of tubes can be achieved with the welding operation, per unit of time, than when using deep-drawing and stretching.